Sputter deposition has become a widespread technique to add functionalities to substantially planar substrates such as film or sheet form materials. In a low-pressure sputter deposition coater, ionised noble gas ions are accelerated towards a negatively biased target. Atoms are kicked out of the target when the noble gas ions impinge on its surface. A magnet system may be provided under the target surface to confine the free electrons in a racetrack. Within the racetrack the ionisation degree of the noble gas is greatly increased and the ion bombardment of the target is therefore more intense below the racetrack. Hence the magnet system defines the area on the target from which atoms are sputtered away. Moreover, under the racetrack, target atoms are ejected mainly in a direction perpendicular to the surface of the target. Such a process is known as magnetron sputter deposition. The knocked-out atoms hit the substrate surface where a dense coating forms. When also reactive gasses are admitted into the coater, reactions with the impinging target atoms will occur at the surface of the substrate, enabling the formation of compound materials such e.g. as oxides or nitrides. In the case of reactive magnetron sputter deposition it has been found to be very advantageous to introduce two targets in the coater that are alternately used as a cathode and an anode. This is known in the art as alternating current (AC) magnetron sputtering—when a sinusoidal current or voltage source is used—or pulsed magnetron sputtering—when a switched direct current (DC) voltage or current source is used.
The functionalities added to the substrate can be diverse. Film-like materials can e.g. be provided with a moisture barrier for packaging applications or can be made electrically conductive with a transparent ITO (Indium tin oxide) coating for use in e.g. touch-screens. Sheet-like materials such as e.g. windowpanes or display glass sheets—for use in a flat panel display—can be coated with an anti-reflective coating or a low-emissivity coating. Magnetic coatings or protective coatings—such as diamond like coatings (DLC)—are applied on disks to provide for the necessary magnetic and protective properties of a hard disk.
Nowadays there is an emerging need to apply a coating on both sides of such planar substrates. The coating on either side can be different or can be equal in nature depending on the use. Some areas where the need for such double-sided coating application is particularly felt are:                Display coaters where an anti-reflective coating is applied on the viewing side of the coater and a transparent conductive oxide can be coated on the other side.        Magnetic hard disks where equal coatings are applied on both sides as both sides are used for magnetically storing and reading data.        Windowpanes where a low-emissivity layered stack is applied on the interiorly oriented side of the glass pane, while a soil-resistant coating is applied on the weather exposed side of the window. Such windows have the particular advantage that they are “self-cleaning” or “easy-cleaning”.The diversity in functionalities added by the sputter coating process is reflected in the plethora of sputtering installation types that are available nowadays. Film-form materials are mostly coated in roll-to-roll web coaters wherein the film is unrolled from a single roll, passes on a central cooled drum around which different sputtering stations are arranged circumferentially before being wound onto a roll again. Some roll-to-roll coaters have a free span section i.e. a section where the film is held taut between two parallel rolls while being coated. Glass for flat panel displays is predominantly coated in vertical display coaters wherein the glass pane is passed in front of one or more sputtering stations by means of a conveyor system that transports the glass pane under a slant angle. Windowpanes are almost exclusively coated in large area glass coaters wherein the horizontally oriented glass sheets pass on rollers underneath different sputtering stations.        
For certain areas, solutions have been proposed to provide a coating to both sides of the substrate in one single pass.                In the field of hard disks, US 2003/0150712 suggests the use of two oppositely oriented separate banks of planar or rotary targets to coat both sides of the disks arranged in a planar substrate holder that passes in between these banks.        In the field of windowpane coating, EP 1179516 B1 describes the idea of a two-sides coating process in which not only the substrate is coated from top to bottom, but also from bottom to top at the same time but in different sections of the coating line, thus eliminating the need of a second passage or the need for a flip-over installation. In this publication it is described that sputtering targets can also be mounted on the bottom of the coating installation as e.g. depicted in FIG. 1.However, the proposed solutions are not always satisfactory. All infrastructures to cool, to energise and to carry the target and the gas feed system must be made in double leading to an ‘installation problem’. In particular for large area windowpane coaters the investment needed to refurbish the installation so as to accept lower target mounting is high. As the total number of layers that have to be deposited is increased and the layers cannot always be deposited at both sides in the same line section, the coating line may have to be lengthened (a costly investment), or the line speed may have to be decreased (reducing the throughput). In addition, the coating target at the lower side of the coating installation is not easily accessible for replacement and servicing.        
A second problem associated with the described art is that although the planar substrates provide some division between the upper and lower part of the sputtering chamber, this separation is not gastight and process gasses are mixed between the upper and the lower part of the chamber (the ‘gas mixing problem’). The layers to be deposited on both sides can therefore not be chosen at will. For example it is not possible to deposit on the top side an oxide such as SiO2 using a silicon containing target and oxygen as a reactive gas, and at the bottom side a Si3N4 coating deposited from a silicon target in a nitrogen atmosphere: the more reactive O2 gas at the top side will disturb the reaction of the less reactive N2 gas at the bottom side. Not only gasses are intermixed, but apparently also the target materials may overspray the margins of the glass pane as explained in EP 1179516 B1.
The inventors therefore set themselves the task to overcome the mentioned problems and to come up with a solution that will now be described in more detail.